I have a inground fibreglass 40 000 litre pool with salt water chlorinator. Because of high levels of chlorine I have been adding around 3 cups of liquid acid to keep the ph level down. My ? is what happens to all that acid thats been added to the pool? I have read my systems require about 1 cup of acid a week to keep the ph down. But what happens to all that acid? Does it build up? Also how should I be adding the acid? I have had 2 different answers, 1 said shallow end other said deep? And how long should I run pump for after adding it. Thanks

Assuming you are adding Muriatic Acid (31.45% Hydrochloric Acid), then all that builds up is chloride ion, part of sodium chloride salt. When you add chlorine over time, chlorine will get converted to chloride ion when it gets used up. However, eventually you will be adding some baking soda since the TA will drop over time.

SWG pools tend to rise in pH at least partly due to the outgassing of carbon dioxide. You can reduce this problem by lowering the TA. You should also have your CYA level at 70-80 ppm and keep your FC at least at 4 ppm to prevent algae (1-3 is NOT sufficient). The higher CYA will protect chlorine breakdown from sunlight letting you lower your SWG ontime setting and that will reduce the rate of pH rise. The TA level can be around 80 ppm. You can also add 50 ppm Borates to your pool either through a combination separately added of 20 Mule Team Borax and Muriatic Acid or via pH-balanced Proteam Supreme Plus (which is mostly Boric Acid). This inhibits algae growth and also is an additional pH buffer.

Finally, if you use a pool cover, that will also help. If you have sources of aeration such as waterfalls, spillovers, fountains, etc., then turning those off or running them less frequently will help.

As for adding acid to the pool, you want to do so very slowly over a return flow in the deep end with the pump running. After you are done, you want to lightly brush the sides and bottom where you added it to thoroughly mix it. This same procedure should be used for most other chemicals, especially bleach or chlorinating liquid.

Thanks for all the info Richard very helpful. New to the pool game, gradually working it out.Just one question, we have a fountain that automatically comes on, what negative effects does this have if it's running all the time apart from water evaporation?

A fountain will tend to aerate the water (improve air-water exchange) and that will tend to ougtas carbon dioxide from the water which will make the pH rise. Pools are intentionally over-carbonated (relative to air) in order to provide a pH buffer and to saturate the water with calcium carbonate to protect plaster. So running your fountain will tend to exacerbate a rising pH problem. Lowering the TA level will help, but turning off the fountain most of the time will also help reduce the rate of pH rise.

Thanks again Richard, I get it now. Another question, I'm a pain aren't I. The pressure gauge on my filter is very low, I have replaced my gauge but it's still low. The whole pool system has only been installed for about 7 months. Could it be the imperella being partly blocked? Everything sounds okay and looks to be working okay.

What effect does it have on the levels when you top up the pool with fresh tap water?

For a partial drain/refill, it simply dilutes the water of everything that is in pool water, but adds to it whatever is in tap water. Typically, tap water has some TA and CH so those won't get diluted as much, but usually tap water isn't really high in CH unless it's from a well.

HOWEVER, in the above, I'm talking about doing a partial drain followed by a refill from tap water. If you are talking about filling up after evaporation, then evaporation concentrates what's in pool water and the tap water dilutes it right back to where it started plus adds what is in the tap water. So the net result is that the TA and CH increase by what is in tap water.

How much does it lower the TA when you add acid each time?

TA goes up and down with acid or base like a yo-yo. To permanently lower TA requires a combination of aeration and acid at lower pH, at least to occur quickly. The procedure is outlined here. If your pool is naturally outgassing carbon dioxide, then adding acid over time will slowly lower the TA. When measuring at two points in time at the same pH, the relationship is that it takes 3.2 cups of Muriatic Acid (31.45% Hydrochloric Acid) to lower the TA by 10 ppm in 10,000 gallons. It takes proportionately more acid in a larger pool; less acid in a smaller pool. However, you do NOT just add the acid in one dose as it would usually lower the pH too much -- follow the procedure I linked to instead.

How long do you have to wait until you can swim after adding acid or other chemicals?

This partly depends on the quality of circulation in your pool, but a half hour is conservative and safe. In my own pool, I've done tests that show that most of the dispersion/mixing occurs in just 5 minutes when the pump is at higher speed (48 GPM with solar panels on) and after about 20 minutes there is no noticeable change in water chemistry parameters.

You should add acid or chlorinating liquid or bleach over a return flow in the deep end with the pump running. Then, for extra safety, lightly brush the side and bottom of the pool where you've added the chemical to ensure thorough mixing. This is especially true for vinyl pools and pools with no floor drain (most above-ground pools) as the circulation near the bottom is quite poor.

If everything appears to be working fine then the problem might be the gauge itself or the opening that leads to the gauge. You can take the gauge off and use a pipe cleaner to clear the opening if clogged.

A replacement gauge is pretty cheap and easy to do if you feel it isn't working.

Vetsa wrote:Thanks again Richard, I get it now. Another question, I'm a pain aren't I. The pressure gauge on my filter is very low, I have replaced my gauge but it's still low. The whole pool system has only been installed for about 7 months. Could it be the imperella being partly blocked? Everything sounds okay and looks to be working okay.